Bituminous water proofing membranes, e.g. in the form of roofing felt, conventionally consist of a carrier or carcass formed by woven, or non-woven fibers which are coated with and possibly impregnated with bitumen (asphalt).
Rag paper is used today as the carcass in the production of roofing felt. Rag paper has the advantage of a relatively low price, and gives the roofing felt an appropriate break elongation for certain applications, but the roofing felt produced has some less good properties, primarily one of taking up water. This results in that the water turns into vapour, which in different places lift the upper bituminous layer of the felt and forms so-called blisters or bubbles. The blisters thus occurring have a tendency to crack and allow further penetration of water. If the surface of the felt is granulated, the granules will fall off to a certain extent due to the formation of blisters, UV protection of the felt thus being reduced and as a result its strength and life also.
Glass fiber therefore appear to be a suitable material for the carcass, due to the unwillingness of the glass fiber material to take up and be affected by water, but carcasses formed from glass fiber have less favourable mechanical properties, e.g. a break elongation of about 2%, which is insufficient, taking into account the movements which the substructure of roofing felt normally has, these movements being transferred to the felt. The result may be that roofing felt cladding formed with a glass fiber carcass cracks. Polyamide fibers have been tried as carcasses for bituminous water proofing membranes, but have been found to be too thermally sensitive in conjunction with manufacturing of the membrane, since asphalt is applied to the carcass at a temperature of about 160.degree. C.
Carcasses made from polyester fibers have been found to be favourable with respect to several mechanical properties, but have unsatisfactory dimension stability in a warm state, e.g. in connection with applying the bitumen, and have a very unfavourable cost in comparison with carcasses from rag paper, glass fiber fabric and the like. Even so, polyester fiber fabric has gained use as carcass for roofing felt. Due to the deficient heat dimension stability of the polyester fiber fabric, the carcass should have a superficial weight of at least 150 g/m.sup.2, preferably 170 g/m.sup.2, for being able to meet stability requirements in conventional manufacture of roofing felt.
It has further been attempted to produce roofing felt carcasses from a mixture of glass fibers and polyester fibers, with the object of avoiding the unfavourable properties of the polyester fiber carcass, but the fiber fabric thus produced has a utilizable elongation determined by the break elongation of the glass fibers, which makes a mixed fiber fabric of this kind unsuitable.
Wood pulp fibers have always been regarded as unusable for carcasses intended for bituminous water proofing membranes. For example, base felt which is to be used as a rag paper carcass for roofing felts, shall have a greatest content of 15% by weight of wood pulp fibers, with mechanical pulp fibers not exceeding 5% by weight.
One object of the invention is to provide a non-woven fabric for use as carcass for bituminous water-proofing membranes, e.g. in the form of roofing felt or sarking felt, this fabric giving the membrane acceptable insensitivity to moisture, acceptable dimension stability during and after manufacture of the membrane and having acceptable elongation and elasticity as well as being relatively thin, i.e. with a superficial weight of 150 g/m.sup.2 or less.
A further object of the invention is to give directions as to the use of a non-woven fabric in accordance with the invention as a bitumen carrier in a water proofing membrane such as roofing felt, where the membrane complies with the requirements applicable to its use.
A still further object of the invention is to achieve a non-woven fabric for the purpose mentioned, the fabric containing a substantial portion of cellulose fibers, particularly plant fibers.